I'm a wit's end here, and seconds away from taking my chances with the authorities and installing a Megasquirt and a set of Jenvey throttle bodies. Alternatively I could hide the injectors in front of the air horns, but that will take a bit of fabrication.

When hot (enough) she will stumble and stall, even when the idle is set as high as 1300 RPM. After it happens, restarting is for all intents and purposes impossible apart from a tow start. Ordinary bump starting will not cut muster. I can catch the stumble with the accelerator, but in traffic that turns into continous heel-and-toe, which is more than just a little tiring. In the garage it took about 15 minutes of idling before she croaked, five minutes of slightly spirited driving will yield the same result.

What is going on here? The inlet manifold is hot and stays hot for a long time.

No wonder carbs disappeared from production vehicles once the alternative was economically viable, at least am I pi&%ed off with them.

Hi Joachim,
I was at Lotusbit's yesterday and talked to Mike Taylor regarding your hunt to find a stealth EFI system. I understand he is working on a system to go inside DHLA's so it might be worth a call.
On your car I take it you have the flexi insulating mountings for the carbs and not gone solid like for FI? Is the car generally running hot?
Gordon

I'll have to ring Mike on Monday and find out more, thanks for the heads up Gordon! I need a crank angle sensor assembly anyway, crank fire ignition is a lot easier to hide than EFI. I am thinking to mount a VW coil pack on top of a gutted distributor, would not stand out since the distibutor is obscured from view. VW part numbers 032 905 106B (wasted spark) or 06A 905 097 (individual coils).

But back to the matter at hand, a bit of differential diagnosis:

1. It is not running hot, dial is mid-gauge and stays there even when idling for a long time. No indication that the oil is overheating either, oil pressure ~2 bar at idle with 5W-40 oil when it is in trouble.

2. Intermittent contact in tachometer or fuse for switched circuit. Seems unlikely since it never misfires when revs > 2000 RPM. I have had intermittent contact at the fuse box before, that behaved rather differently. TODO: verify that I actually have spark when cranking after it dies.

3. Stale fuel. Fuel is from last year, but it did the same thing last year when the fuel was fresh. Also, expected outcome of stale fuel is hard starting and poor idle, especially when cold. Neither seems to be true.

4. Overheating Pertronix: Not probable, since enough revs will restart it. Electronics generally cool down rather quickly, so does not explain trouble restarting an hour after it stalled.

5. Fuel frothing: Not probable. Idle is nice and smoothish, carbs mounted with rubber insulator washers and plastic spacer plates as supplied by Mike when I bought the engine and carbs. No noticable extra shaking before stall.
Carbs are Spec 5, btw.

6. Heat soak problem in carbs: so far the most probable cause. It explains the time constant, inlet manifold is hot to touch for a long time after driving.

I need to install a WBO2 to see if mixture is moving quickly or slowly before it stalls.

I had something similar years ago. IIRC, my fuel filters were clogged. Enough fuel would seep thru the fuel filters when the car was sitting for a period of time but the engine would die after a while from fuel starvation.

Also, what changes with heat on your carb setup? My Zenith Strombergs have an air bypass valve that would open when hot and cause rough idling.

I also think your idea of the electronic ignition failing when hot is a possibility. Do you have a way to check if you are getting a good spark when this happens?

Fuel filter has fuel, fuel pump cadence sounds ok. Just before stalling it bogged and recovered a few times, and fired a few shots in the exhaust.

I might try opening up the idle screws a bit more, I just remember that I turned them in a bit last summer after the biannual inspection. They said it was "a bit too rich" in a way that told me it was slobbering rich at idle, the limit is 3.5% CO.

But it still really bothers me that I can't seem to get it started for hours after it stalls.

I had similar symptoms and finally discovered it to be a faulty (new) rotor. I replaced it and a short time later it started happening again. Quite a bit written about the substandard rotors breaking down with temperature. I finally tried a red rotor from "Distributor Doctor" who claims their red rotor solves this issue and have not had any recurrence.
I had tried new coil, cap, pertronix and distributor before discovering the rotor problem.
Hope this helps.
Jim

Good point, Jim. I had two black rotors fail from subjecting them to MSD 6A ignition and high energy coil. They burned a carbon track straight down through the center to the shaft! They never worked again when cooled off though.
No problems with the Advanced Distributors red rotors.

Are your Dellortos stock Jensen-Healey spec? And are they emissions or non-emissions? What is the elevation above sea level where you live?

I have three 907s equipped with 45E Dellorots, and have never experienced percolation/ vapor lock problems with any of them. And one of them is in a mid-engined Esprit where there's little airflow, and the engine bay gets hotter than typical front engine cars.

On this side of the pond, Zenith-Strombergs were stock on the 907, and they did suffer from vapor lock; but the Dellortos cured all of that for me. So I'm surprised to hear that you're having trouble. I suspect it's something about your jetting set-up, your home elevation (mixture goes richer at higher elevation), or a failure in your carbs, but I'm not ready to blame carbs... or Dellorto carbs.

I'm aware of those, I believe Patton Engineering makes them. But for a straight four with the federal spec intake manifold, that solution is not one I would touch with a ten foot pole. Fuel distribution will be a major headache with that induction system. Strombergs don't look cool either... :-)

subwoofer wrote:3. Stale fuel. Fuel is from last year, but it did the same thing last year when the fuel was fresh. Also, expected outcome of stale fuel is hard starting and poor idle, especially when cold. Neither seems to be true.Do you have oxygenate fuel in Norway? Ethanol in the fuel? Oxygenated fuel goes bad very quickly. 2.5 to 3 months is more than enough time for it to go bad, and to gum-up the carbs in the process.

subwoofer wrote:5. (Snip)...
Carbs are Spec 5, btw.Okay, I should have read on before posting my first reply. Spec 5 calls for 10 gram floats set to 16.5 - 17.0 mm height. Check that. The weight in grams, or the part number are molded into the float. Some float weights are hard to come by now, or just unobtainium. If a different weight float has been installed, then the float height must also be changed to a level appropriate for the weight used. The other weight options are (were ?) 8.5 and 7.0 grams. If either of them were substituted without adjusting the height, then an overall lean condition will result.

subwoofer wrote:6. Heat soak problem in carbs: so far the most probable cause. It explains the time constant, inlet manifold is hot to touch for a long time after driving.What temperature thermostat is installed?

Fortunately, no alcohol in standard fuel here in Norway, at least that I know of, and E85 flopped. I think there is a grand sum total of 8 - eight - filling stations in the whole country you can still get it.

I think the termostat is for 88C, it could possibly be a 92C one. It never gets really hot here, a summer heat wave means 30C (85F) maximum daytime temperature for about a week.

What I really can't understand is why the situation is deteriorating. She ran very fine the year before last, then started acting up last year and is impossible this year. But Kurt may have a point, it could be ignition related. The problems carry more than a little resemblance to the way she was acting when I still had points in the distributor and I accidentally had kept the ballast resistor when installing a fresh Lucas Sport Coil (12V). Unless Lucas is still living up to its reputation I have no reason to believe the fault is in the coil, it has less than 2000 miles on it. But rotor and cap need to be checked. Wiring in general also needs to be checked, and a relay for pulling coil current more or less straight from the battery could be installed. I have a spiyda tach conversion kit lying around, just haven't gotten around to installing it yet, so getting coil current out of the tach could be imminent.

This week and next are just too busy to work on the car. In addition to a day job, there is family to attend to and I'm playing three concerts with a jazz big band in as many days this weekend.

Oh, that is very close to a solution, but unfortunately the front and rear bolt patterns are not identical on the DCOE. You can't bolt it on to the front of the carbs and have them flow the right direction. Apart from that, it is a very good idea! Cook up a set of injector holder rings that bolt onto the front of the carb, could be easier than going Ducati style.

The first step to a good carb tune-up is to give the ignition a good tune-up, so don't give up on that. But I wouldn't be surprised if a good carb cleaning didn't help. Even non-oxygenated fuel makes varnish deposits during long Winter storage, and like any dirt & dust accumulation, the effect builds up over time.

Hi Tim,
I have been running the Jenvey 48 TBs with good results for the past couple of years. I got the kit from Mike Taylor at Lotusbit's and I was a bit concerned about going from 45 to 48mm but the improvements in drivability, economy and emissions is notable. Emissions are only a problem at the annual MOT time.
Joachim is looking for an EFI solution that keeps the standard appearance - hence the suggestion of the short TBs inside the Éclat air box, but as he says the bolt pattern between inner and outer faces are different.
Gordon

Update: I think this is boiling down to a fuel pump issue (pun intended).

I have been thinking that the fuel pump may have been sounding just a bit funny, so I switched on the ignition while watching the fuel filter I installed just before the carbs. It was about half full after sitting for a few days, when I turned on the pump air bubbles filled the glass!

I still can't explain why it runs nice at load and RPM but falls on its face at idle, but at least I have found a problem that needs fixin'.